1. Field of the Invention
The present invention relates to a method of manufacturing semiconductor devices and, more particularly to a method of manufacturing thin film gate insulated field effect transistors consisting of polycrystalline semiconductor films.
2. Description of the Prior Art
The prior art includes methods utilizing low pressure CVD by which polycrystalline semiconductor thin films are deposited at temperatures ranging from 550xc2x0 C. to 900xc2x0 C. Recently, along with development of liquid crystal panels having wide display area, there have been needs for the deposition technique to coat polycrystalline semiconductor films over with areas of substrates.
Polycrystalline films are formed by depositing amorphous semiconductor films by low pressure CVD and then recrystallizing the amorphous films since direct deposition of polycrystalline films on wide areas is difficult for the reason of reaction temperature. On the other hand, it is very difficult to deposit uniform semiconductor films by low pressure CVD. This is the problem also in the case of plasma CVD which, requires longer deposition times.
Sputtering on the other hand is excellent in this sense. Particularly, there are following advantages when films are deposited by magnetron sputtering.
1) The surfaces of substrates are less damaged by high energy electrons since the electrons are confined in the vicinity of the target.
2) Wide areas can be coated at lower temperatures.
3) No dangerous gas is used so that safety process and practicability are ensured.
The sputtering is carried out without hydrogen doping because the electric characteristics of hydrogenated amorphous semiconductors deposited by sputtering are not so good as to satisfy requirements of channel formation for transistors. The semiconductor films deposited by the sputtering, however, have a disadvantage that thermal crystallization thereof is very difficult.
It is an object of the present invention to provide a method of manufacturing gate insulated field effect transistors consisting of polycrystalline semiconductor films in which barrier heights of grain boundaries are substantially decreased.
It is another object of the present invention to provide a method of manufacturing gate insulated field effect transistors consisting of polycrystalline semiconductor films over a wide area of a substrate.
It is a further object of the present invention to provide a method of manufacturing gate insulated field effect transistors consisting of polycrystalline semiconductor films which are so dense as not to easily form natural oxide within the films.
If is a still further object of the present invention to provide a method of manufacturing gate insulated field effect transistors consisting of polycrystalline semiconductor films which possess lattice distortion.
Additional objects, advantages and novel features of the present invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the present invention. The object and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve to foregoing and other object, and in accordance with the present invention, as embodied and broadly described herein, a semiconductor film to form a channel is deposited on a substrate by sputtering in an atmosphere comprising hydrogen whereas an oxide film to form a gate insulating film is deposited by sputtering in an atmosphere comprising oxygen. The semiconductor film and the oxide film are deposited on the substrate one after the other in separate sputtering apparatuses in order to prevent oxygen from leakage into the apparatus for deposition of the semiconductor film from the other apparatus for deposition of the oxide film and vice versa. Irrespective of which film is first deposited and the other next, the density of oxygen atoms occurring in the semiconductor film is limited to no higher than 7xc3x971019 cmxe2x88x923, preferably no higher than 1xc3x971019 cmxe2x88x923.
In order to obtain a gate insulated field effect transistor having high performance two or more kinds of layers constituting the transistor may be deposited by sputtering in individual sputtering apparatuses. Two or more of the sputtering apparatuses may be connected so that transportation of a substrate of the transistor can be performed without exposing the substrate and layers formed thereon to the air. For example, gate valves or subsidiary chambers may be provided between the sputtering apparatuses. In the case where the sputtering apparatuses are arranged in series, a gate insulated field effect transistor can be formed on an assembly line by the use of such apparatuses so that productivity thereof can be made high. Two or more kinds of layers constituting the transistor may be formed on an assembly line by sputtering in sputtering apparatuses arranged in series.
In accordance with a preferred embodiment of the present invention, the semiconductor film is first deposited in an amorphous phase or an equivalent phase. The amorphous semiconductor is then given thermal treatment at 450xc2x0 C. to 700xc2x0 C. typically at 600xc2x0 C. in order to convert the amorphous phase to a polycrystalline phase at lease in the channel region. In stead of the thermal treatment, the amorphous semiconductor may be radiated with a beam emitted from a light source, for example a laser or a halogen lamp in order to convert the amorphous phase to a polycrystalline phase at lease in the channel region. This recrystallization easily takes place as compared to the conventional case without the use of hydrogen introduction as explained supra. This is considered because of the following reason. In the conventional case, amorphous semiconductors such as a-Si are deposited to form a certain type of microstructure in which distribution of silicon atoms is uneven. This microstructure hinders the progress of recrystallization during thermal treatment or radiation of a beam emitted from a light source. The inventors have been confirmed that the formation of the microstructure is prevented by introducing hydrogen into the semiconductor film which can be easily recrystallized by thermal treatment or radiation of a beam emitted from a light source. The average diameter of polycrystals formed after the thermal treatment or radiation of a beam emitted from a light source is of the order of 5 xc3x85 to 400 xc3x85, typically 50 xc3x85 to 200 xc3x85. Such small size of grains in the polycrystals is particularly effective to prevent reverse current leakage across N+-I and P+-I junctions. The proportion of hydrogen introduced into the film is no higher than 5 atom %. An important feature of the semiconductor film is lattice distortion which enables close connection between polycrystals at interfaces thereof. This feature helps to lessen discontinuity at the interfaces and hinder the formation of barriers at the interfaces while, in the case without such lattice distortion, impurity atoms such as oxygen tent to be collected at the interfaces and form crystal barriers which hinder transportation of carriers. In addition to this, since the oxygen density in the semiconductor film is no higher than 7xc3x971019 cmxe2x88x923 in accordance with the present invention, potential barriers are substantially not formed. Such low oxygen density can be realized by carrying out deposition of oxide films and semiconductor films in separate chambers provided for exclusive use in depositing the respective films. The oxygen density can be further decreased by evacuating the inside of the chamber to a very high vacuum condition, in advance of deposition, by means of combination of a turbo molecular pump and a cryosorption pump. Because of this, the mobility (field mobility) of electron in the semiconductors formed in accordance with the present invention is improved as high as 50 to 300 cm2/V.S.
Furthermore, the semiconductor deposited by sputtering is so fine as not to allow oxidation to reach to the inside of the film and only a very thin oxide films are formed at the surface thereof while the semiconductor deposited by plasma CVD includes a relatively high proportion of its amorphous phase along which oxidation proceeds into the inside of the semiconductor. This fine structure of the present invention helps to reduce interfacial barriers between crystals in association with lattice distortion.
The atmosphere in which sputtering for deposition of the semiconductor film is carried out may be hydrogen, a mixture of hydrogen and an inert gas such as Ar and He, or a hydrogen compound which does not change the property of the semiconductor film such as SiH4 or Si2H6. Anyway, the density of hydrogen in the atmosphere is important to introduce appropriate hydrogen into the semiconductor film and cause sufficient lattice distortion therein. In the case of hydrogen/argon mixtures, the hydrogen proportion is selected between 5% and 100% (therefore the argon proportion between 95% and 0%), typically between 10% and 99% (argon between 1% and 90%), desirably between 25% and 95% (argon between 75% and 5%).
In accordance with preferred embodiments, an oxide film forming a gate insulating film and a semiconductor film forming a channel region are deposited successively so that the electrical characteristics of the gate insulated field effect transistor formed from the films become stable without being influenced from external disturbances. Of course, the present invention can be applied to a variety of other type transistors such as staggered types, coplanar types, inverted staggered types, inverted coplanar types.